You can't GPU fold in Linux. No Client for that - yet.
HDDs have no effect on ppd.
Like Sugar said, 2P / 4P rigs rely on their power to complete WUs faster than anticipated. As such, a bonus was instated for SMP WUs that is exponential. Any GPUs that you add (and their associated PPD) will negate the SMP bonus that the CPUs are able of.

your best run would be Linux for the duration of the CC and then you can switch to Win7 for more mundane purposes, as media server, shared storage, occasional gaming and stuff as I do with my 2P folder.